Conventional fiber optic communication systems are well-developed for transmitting high-data rate signals, such as 10 Gbps and 40 Gbps signals. However, these high data rates are being pushed towards ever increasing speeds due to rapid growth in networks. For example, current standards bodies (e.g., IEEE) are considering data rates of 100 Gbps, which would require ever higher transmission rates (i.e., approximately 112 Gbps), once forward error correction (FEC) and framing (e.g., G.709) overheads are considered. Other standards bodies, such as ITU-T, are considering data rates of 120 Gbps, again requiring even higher transmission rates (i.e., approximately 130 Gbps). Such high data rates are beyond the limit of conventional electronics and optics. For example, conventional systems utilize a direct binary modulation scheme. Disadvantageously, direct binary modulation schemes have poor spectral efficiency which limits overall transmission system utilization in wavelength division multiplexed (WDM) systems.
One method for improving spectral efficiency and reducing the demands on system electronics and optics is to use both orthogonal polarizations of optical signals in single-mode fibers (SMFs), i.e., polarization multiplexing (PolMux). This effectively reduces the signal baud rate by a factor of two by transmitting two data streams on the same carrier signal. Signal transmission properties can also be improved by using Differential Phase Shift Keying (DPSK), which provides both enhanced tolerance to optical amplified spontaneous emission (ASE) noise, and an enhanced tolerance to deleterious fiber non-linear effects.
A further improvement in spectral efficiency, while preserving other advantages, can be achieved using Differential Quadrature Phase Shift Keying (DQPSK) transmission. At the same time, tolerance to chromatic dispersion (CD) is increased and the baud rate of the signal is also decreased (but not the bit rate). For example, a 112 Gbps signal can be transmitted using PolMux and DQPSK encoding as a 28 GBaud signal, correspondingly requiring electronic and optical components that need to support only a 28 Gbps bit rate. Advantageously, this allows for high-data rate signal transmission exceeding the limitations of conventional direct binary modulation schemes.
Of note, polarization multiplexed systems require a receiver architecture configured to perform polarization demultiplexing and polarization mode dispersion (PMD) mitigation. PMD is a generally deleterious effect experienced by high-bit rate optical signals as they propagate in fibers. It is related to a deviation in fiber geometry away from an ideal perfectly circular symmetry. The optical signal is correspondingly split into two polarization modes (Principal States of Polarization—PSP), which propagate with different speeds, i.e., the fiber becomes birefringent and looks like a very large collection of waveplates, due to the fiber geometry.
At some point, there is sufficient delay that accumulates between the modes such that the signal arriving at the receiver looks like two streams with different arrival times. The received signal experiences dual-path interference. The delay is independent of the signal data rate, and is an intrinsic property of the fiber optic link. However, the impairment experienced by the data signal is proportional to the bit rate, as the bit period is shortened. PMD poses a particular problem with polarization multiplexing as data is carried on orthogonal polarizations, and PMD destroys the orthogonality and induces polarization cross-talk. Accordingly, PMD mitigation is a required component in high-speed receiver architectures for polarization multiplexing.
Referring to FIG. 1, the current state of the art for a polarization multiplexed/DxPSK receiver system 10 uses separate building blocks 12,14,16 for the functions required to implement PMD mitigation 12, polarization demultiplexing 14,16, and DPSK/DQPSK (collectively referred to as “DxPSK”) signal demodulation and balanced detection 22,24,26,28. Disadvantageously, conventional transmission schemes have increased complexity associated with receiver design. For example, an input 28 includes an optical signal with two polarizations. The input 28 is connected to a PMD compensator 12 shared for both polarization states. A controller 18 is utilized to provide feedback and control between the PMD compensator 12 and a polarization controller (PC) 14.
In particular, the system 10 is required to separate orthogonal polarizations with a high degree of polarization cross-talk rejection at a polarization beam splitter (PBS) 16. Also, PMD tolerance is degraded as it destroys orthogonality between polarizations. Finally, DxPSK modulation requires a separate Delay Demodulator 20,22 for each Quadrature and polarization (i.e., two for DPSK and four for DQPSK), which correspondingly increases system cost. Treating each stage as an independent PMD Compensator, followed by PM demultiplexing, followed by DxPSK demodulation is rather expensive, inefficient, and would require faster control loops at each stage.
Thus, it would be highly advantageous to provide a receiver scheme that can achieve PMD mitigation, polarization demultiplexing, and DxPSK signal demodulation and balanced detection in a single system with a reduced part count.